feat(iOS): Enable synchronous header subview shadow state updates by default (#3933)

## Description

This PR changes the default value for
`synchronousHeaderSubviewUpdatesEnabled` flag to `true`


## Changes

Updated flag defaults, native & JS initialization

## Before & after - visual documentation

N/A

## Test plan

See #3282 

## Checklist

- [ ] Included code example that can be used to test this change.
- [ ] For visual changes, included screenshots / GIFs / recordings
documenting the change.
- [ ] For API changes, updated relevant public types.
- [ ] Ensured that CI passes

---------

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

Author: kmichalikk

FabricExample/App.tsx

@@ -3,7 +3,7 @@ import { featureFlags } from 'react-native-screens';
 
 featureFlags.experiment.synchronousScreenUpdatesEnabled = false;
 featureFlags.experiment.synchronousHeaderConfigUpdatesEnabled = true;
-featureFlags.experiment.synchronousHeaderSubviewUpdatesEnabled = false;
+featureFlags.experiment.synchronousHeaderSubviewUpdatesEnabled = true;
 featureFlags.experiment.androidResetScreenShadowStateOnOrientationChangeEnabled =
   true;
 featureFlags.experiment.iosPreventReattachmentOfDismissedScreens = true;

ios/RNSScreenStackHeaderSubview.mm

@@ -116,6 +116,7 @@ - (instancetype)initWithFrame:(CGRect)frame
     static const auto defaultProps = std::make_shared<const react::RNSScreenStackHeaderSubviewProps>();
     _props = defaultProps;
     _lastScheduledFrame = CGRectZero;
+    _synchronousShadowStateUpdatesEnabled = YES;
   }
 
   return self;

src/fabric/ScreenStackHeaderSubviewNativeComponent.ts

@@ -14,7 +14,7 @@ export type HeaderSubviewTypes =
 export interface NativeProps extends ViewProps {
   type?: CT.WithDefault<HeaderSubviewTypes, 'left'>;
   hidesSharedBackground?: boolean | undefined;
-  synchronousShadowStateUpdatesEnabled?: CT.WithDefault<boolean, false>;
+  synchronousShadowStateUpdatesEnabled?: CT.WithDefault<boolean, true>;
 }
 
 export default codegenNativeComponent<NativeProps>(

src/flags.ts

@@ -1,6 +1,6 @@
 const RNS_SYNCHRONOUS_SCREEN_STATE_UPDATES_DEFAULT = false;
 const RNS_SYNCHRONOUS_HEADER_CONFIG_STATE_UPDATES_DEFAULT = true;
-const RNS_SYNCHRONOUS_HEADER_SUBVIEW_STATE_UPDATES_DEFAULT = false;
+const RNS_SYNCHRONOUS_HEADER_SUBVIEW_STATE_UPDATES_DEFAULT = true;
 const RNS_ANDROID_LEGACY_TOP_INSET_BEHAVIOR_DEFAULT = false;
 const RNS_ANDROID_RESET_SCREEN_SHADOW_STATE_ON_ORIENTATION_CHANGE_DEFAULT =
   true;